Remember all of those supply vendors you pass by? Well look at their inventory and always check the last page. There’s usually some limited supply recipes that you can buy. The ones that are in weird places are even better. The goblin in Moonbrook sells rage potion recipes, there are 2 goblins in Duskwood, the one on raven hill sells shadow oil recipes and the one on the road north out of Duskwood sells goblin jumper cables. All of these recipes sell for 200-1000% profit in the auction house.
Want to know a secret? Buy 5 different tuxedo patterns from the tailor in Ironforge for around 40 silver each, then turn around and sell them in the auction house for 2 WOW Classic gold or more! There are also 2 thorium recipes off of the engineering supplier in the gnome area that sells for pretty good. People are just lazy or don’t know that vendors sell these recipes. Keep in mind they’re all limited sell so if a recipe isn’t there check back in 20 minutes.
Also in Moonvale is the arcanite rod enchantment and some other high-level recipes. All of them can be purchased for 1-2 gold and buyout in auction for 5-7 gold. Once you have your recipes hearthstone back to Auberdine and drop them in the mail to your mule guy in front of the auction house.
If you do it right you should have about 20 auctions going at a time and it doesn’t draw suspicion like putting up 10 auctions all for the same item (a great way to kill your profits by the way). I make about 20-30 gold a day from about 30 minutes of work. The best part is you can do it at any level.
Some other good places for recipes, Ashenvale has a ton of good stuff at Astranar and that other post. You can get the expert cooking book and the big bear steak recipes (the bear recipes cost like 20 silver and you can sell it for 1 gold a lot of times, and it’s unlimited supply). The Wetlands has a vendor in the middle of the map. Stormwind in the old town district has a pub and upstairs a guy sells about 20 different cooking recipes, many of the same ones you have to quest for.